Liquid donor development with electrophoretic cleaning

ABSTRACT

AN ELECTROSTATOGRAPHIC IMAGING MEMBER BEARING AN ELECTROSTATIC LATENT IMAGE ON A RECORDING SURFACE IS BROUGHT INTO CONTACT WITH A TONER LAYER ON THE SURFACE OF A DONOR MEMBER IN A LIQUID DEVELOPER BATRH, SEPARATED SLIGHTLY FROM THE DONOR MEMBER AND THEREAFTER SUBJECTED TO AN ELECTRIC FIELD WHICH CAUSES TONER PARTICLES IN THE BACKGROUND AREAS OF THE RECORDING SURFACE AS WELL AS TONER PARTICLES SUSPENDED IN THE SPACE BETWEEN THE RECORDING SURFACE AND THE DONOR MEMBER TO ELECTROPHORETICALLY MIGRATE TO THE DONOR MEMBER. THE DEVELOPED IMAGE ON THE RECORDING SURFACE PASSES THROUGH THE ZONE SUBJECTED TO THE ELECTRIC FIELD TO FURTHER CLEAN THE RECORDING SURFACE. SUBSEQUENTLY A UNIFORM LAYER OF TONER IS ELECTROPHORETICALLY DEPOSITED ON THE DONOR.   D R A W I N G

July 13, 1971 SATORU HQNJO EI'AL 3,592,678

LIQUID DONOR DEVELOPMENT WITH ELECTROPHORETIC CLEANING Filed March 14,1969 INVEN'IURS SATORU HONJO YASUO TAMAI BY MASAMICHI SATO mat X4ATTORNEY United States Patent Office 3,592,678 Patented July 13, 1971U.S. Cl. 117-37 Claims ABSTRACT OF THE DISCLOSURE An electrostatographicimaging member bearing an electrostatic latent image on a recordingsurface is brought into contact with a toner layer on the surface of adonor member in a liquid developer bath, separated slightly from thedonor member and thereafter subjected to an electric field which causestoner particles in the background areas of the recording surface as wellas toner particles suspended in the space between the recording surfaceand the donor member to electrophoretically migrate to the donor member.The developed image on the recording surface passes through the zonesubjected to the electric field to further clean the recording surface.Subsequently a uniform layer of toner is electrophoretically depositedon the donor.

BACKGROUND OF THE INVENTION This invention relates to imaging systems,and more particularly, to an improved method and apparatus fordeveloping electrostatic latent images with a liquid developer.

Processes for the formation and development of images on the surface ofphotoconductive materials by electrostatic means is well known. Theseprocesses include dry techniques such as cascade, powder cloud andmagnetic brush processes and wet techniques such as the liquiddevelopment process. One conventional liquid development processinvolves placing a uniform electrostatic charge on a photoconductiveinsulating layer comprising zinc oxide powder and a resinous bindercarried on a conductive paper substrate, exposing the layer to alight-and-shadow image ot dissipate the charge on the areas of the layerexposed to the light and developing the resulting electrostatic latentimage by depositing on the image a charged toner which is dispersed inan insulating liquid. The charged toner may be suitably colored and mayhave a polarity of charge identical or opposite to that of the latentimage to be developed. If the polarity or charge of the toner isidentical to that of the latent image, reversal de velopment will occurwhereas a toner having a charge opposite to that of the latent imagewill be attracted to the latent image.

Although considered a highly desirable technique for the formation ofimages, difficulties are encountered with attempts to form high qualityimages with the liquid development process. Generally, liquiddevelopment is effected by either immersing the electrostatic latentimage bearing surface into the liquid developer or contacting the imagebearing surface with a uniform film of liquid developer carried on anapplicator surface. The liquid developer adhering to the imaging surfaceis thereafter dried by mere exposure to ambient air at room temperature,circulating heated air, infrared heating, pressure from squeegee rollersor the like. Unfortunately, the liquid developer adhering to thebackground areas of the imaging surface contain toner particles whichremain on the imaging surface after drying. These undesirable backgrounddeposits are particularly acute in high speed continuous tonedevelopment systems employing liquid developers containing highconcentrations of toner particles.

It has been found that background toner deposits can be reduced byrinsing imaging surfaces immediately after development with a liquid aliquid which contains little or no toner particles. However, a rinsingstep presents additional problems in high speed development systemsbecause treatment time and equipment complexity is increased. Inaddition, as a result of increasing toner concentration in the rinsingliquid, additional equipment is required to remove the accumulated tonerparticles. Since most liquid development techniques are deficient in oneor more of the above areas, there is a continuing need for improvedimaging processes.

SUMMARY OF THE INVENTION It is therefore, an object of this invention toprovide an imaging system overcoming the above noted deficiencies.

It is another object of this invention to provide an imaging techniquewhich improves electrostatographic image quality.

It is a further object of this invention to provide an imaging techniquewhich forms images more rapidly.

It is still another object of this invention to provide a more compactimaging apparatus.

It is another object of this invention to provide images having reducedbackground deposits.

It is a further object of this invention to provide an imaging systemsuperior to those of known systems.

The above objects and others are accomplished by providing an imagingsystem in which an electrostatographic member bearing an electrostaticlatent image on its recording surface is brought into contact with atoner layer on the surface of a donor member in a liquid developer bath,then separated slightly from the donor member and thereafter subjectedto an electric field which causes toner particles in the backgroundareas of the recording surface as well as toner particles suspended inthe space between the recording surface and the donor member toelectrophoretically migrate to the donor member, The migration of tonerparticles to the donor member replenishes toner material consumed duringdevelopment. Since toner material is also removed from the liquiddeveloper trapped in the space between the recording surface and thedonor member, background deposits on the recording surface is furtherreduced by the rinsing effect of the substantially toner free trappedinsulating carrier liquid. Surprisingly, the electric field does notcause destruction of the toner image by removal of toner particles fromthe toner image.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages of the improvedelectrostatographic imaging system of this invention will become evenfurther apparent upon consideration of the following disclosure of theinvention, particularly when taken in conjunction with the accompanyingdrawing illustrating a schematic sectional view of anelectrophotographic imaging apparatus employing the donor member andelectric field applying means of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawing,reference character 1 designates a developer container containing aliquid developer 2 comprising toner particles suspended in an insulatingliquid. An endless donor belt 3' supported by drive rollers 32 and 33 issuspended in the liquid developer '2. For purposes of illustration only,a positively charged toner layer is carried on the surface of donor belt3. A photoreceptor web 4 carrying a negatively charged electrostaticlatent image is brought into contact with donor belt 3 by means ofrollers 41 and 42. The photoreceptor web 4 is separated from endlessdonor belt 3 with the aid of a split roller 43 which contacts only theedges of photoreceptor web 4. Photoreceptor web 4 is withdrawn from thebath of liquid developer 2 by means of roller 44 and squeeze rollers 45and 46. The surface of squeeze roller 46 is imparted with a mirrorfinish to promote the appearance of the developed recording sur face ofphotoreceptor web 4. An insulated block is disposed between the upperand lower portions of endless donor belt 3 as well as between dryrollers 32 and 33. An electrically grounded development electrode 51 issecured to the insulated block 5 and positioned in sliding contact withmost of that portion of the rear surface of donor belt 3 which is incontact with photoreceptor web 4 during development. Electrodes '52 and53 are positioned on the rear surfaces of photoreceptor web 4 and donorbelt 3, respectively, in the zone where photoreceptor 4 and donor web 3are maintained in a slightly spaced apart relationship. Electrode 52 iselectrically grounded and electrode 53 is supplied with a negativeelectric potential. A'fter separation of the endless donor belt 3 fromphotoreceptor web 4, endless donor belt 3 is transported around roller33 and between an electrode 54 secured to the lower surface of insulatedblock 5 and grid electrode 55 positioned substantially parallel to boththe endless donor belt 3 and electrode 54. A negative potential isapplied to electrode 54 and a positive potential is applied to gridelectrode 55. A pump 6 driven by a motor 7 is employed to withdrawhighly concentrated liquid developer through conduit 61 from the bottomof developer container 1 and deliver the concentrated liquid developerthrough grid electrode 55 by means of perforated supply unit 62.

When the above described apparatus is in operation, all of the rollersare rotated at appropriate speeds to drive the endless donor belt 3 andphotoreceptor web 4 at an equal rate of speed. The toner layer 31carried on the surface of donor belt 3 is brought into contact with anelectrostatic latent image bearing surface of photoreceptor web 4 bymeans of rollers 32 and 41, respectively. Since the driving means forphotoreceptor web 4 and donor belt 3 are synchronized, there is norelative movement between the surface of photoreceptor web 4 and donorbelt 3 while they are in contact with each other. During the period ofcontact, the toner in toner layer 31 is electrostatically adsorbed onthe electrostatic latent image bearing surface of photoreceptor web 4.The development electrode 51 positioned on the rear side of donor belt 3promotes rapid development and avoids the edge effect problem. Uponcompletion of development, the photoreceptor web 4 is passed betweenrollers 42 and 43 and is separated from the donor belt 3. Immediatelyafter separation, photoreceptor web 4 is maintained in adjacent spacedrelationship with donor belt 3 by means of rollers 43 and 44. Anelectric field between endless donor belt 3 and photoreceptor web 4 isestablished by means of electrodes 52 and 53. This field causes tonermechanically adhering to the background areas of the developed surfaceof photoreceptor web 4 and toner particles suspended in the developerliquid trapped between electrodes 52 and 53 to migrate in the directionof electrodes 53 and deposit on donor belt 3 to form a toner layer 31thereon. The potential difference between electrodes 52 and 53 dependsupon several factors including the potential difference between theelectrodes and the speed at which the photoreceptor web 4 istransported. Satisfactory results are obtained with a potentialdifference of about 100 volts and a distance between the electrodes ofabout 2 millimeters. Since substantially all of the toner particlessuspended in the liquid developer 2. trapped in the area betweenelectrodes 52 and 53 are removed, the toner free liquid in this areapromotes removal of toner particles from the background areas of thedeveloped surface of 4 photoreceptor web 4. The photoreceptor web 4 isthereafter conveyed between squeeze rollers 45 and 46. Since squeezeroller 46 has a mirror finished surface, the possibility ofphotoreceptor web 4 breakage is eliminated.

Upon separation from the photoreceptor web 4, donor belt 3 is conveyedaround roller 33 and into sliding contact along the lower surface ofelectrode 54. During the period in which donor belt 3 is passed overelectrode 54, an electric field is established between electrode 54 andgrid electrode to promote deposition of toner particles from liquiddeveloper 2 onto endless donor belt 3 to form a uniform toner layer 31.To promote rapid deposition of toner particles on donor belt 3, liquiddeveloper 2 is delivered by pump 6 through a perforated supply unit 62into the space between grid electrode 55 and donor belt 3. After asufficient quantity of toner particles is deposited on donor belt 3 toform toner layer 31, the donor belt 3 is again brought into contact withphotoreceptor web 4 to develop additional electrostatic latent images.The development electrode 51 is extended within insulating block 5between electrodes 53 and 54 to shield these latter electrodes therebypreventing toner from being deposited thereon.

Although the embodiment depicted in the drawing illustrates a system inwhich the donor belt 3 retains a negative toner image upon separationfrom photoreceptor web 4, this toner layer may be removed if desired.Thus, a cleaning brush or other suitable means may be positionedadjacent roller 33 to remove the toner layer from the surface of donorbelt 3 for redispersal into liquid developer 2.

Any suitable material may be employed in the donor belt and thephotoreceptor web of this invention. The material should resistdissolving or swelling when contacted with the liquid developer.Preferably, the donor belt material should possess a high electricresistance and a low dielectric constant. Typical donor belt materialsinclude cellulose, triacetate, Teflon, polyethylene terephthalate andthe like.

Any conventional insulating liquid developer may be employed in thesystem of this invention. For optimum results, toner particles having anaverage particle diameter of less than about 1 micron are preferredbecause removal of toner particles from the developed image areas issubstantially eliminated during the cleaning operation in which tonerparticles are cleaned from the background areas. This peculiarcharacteristic of toner particles having an average particle size ofless than 1 micron renders the system of this invention highly suitablefor forming multicolored images on the same surface of a photoreceptorweb because disruption of the toner image does not occur.

Although positive images are formed in the specific example describedabove, it is apparent that reversal images may be obtained by reversingthe polarity of toner material or by reversing the polarity of theelectric potential applied to the electrodes. Obviously, one may apply apotential difference to the development electrode if desired. Asdescribed above the apparatus of this invention includes a developingunit and a cleaning unit incorporated within a single housing. Further,the technique of this invention is adapted to rapidly form a toner layeron a donor member for reuse. Thus, the compact developing system of thisinvention forms low background toner images at high processing speedswith very little toner consumption.

Although specific materials and conditions are set forth in theforegoing examples, these are merely intended as illustrations of thepresent invention. Various other suitable photoreceptor materials,liquid developers and voltage potentials including those listed abovemay be substituted for those in the specific example with similarresults. Other materials may also be added to the liquid developer orphotoreceptor to sensitize, synergize or otherwise improve the imagingproperties or other desirable properties of the system.

Other modifications of the present invention will occur to those skilledin the art upon the reading of the present disclosure. These areintended to be included Within the scope of this invention.

What is claimed is:

1. A method of forming images comprising providing an electrostaticlatent image on a recording surface of an electrostatographic imagingmember, contacting the said recording surface With a uniform toner layercarried on the surface of a donor member immersed in an insulatingliquid developer bath comprising an insulating carrier liquid andsuspended toner particles, separating said recording surface from saidtoner layer thereby forming an imaged recording surface having imageareas and background areas, positioning said recording sur face adjacentto and spaced from said toner layer thereby forming an open cleaningzone in said insulating liquid developer bath between said recordingsurface and said toner layer and providing an electric field across saidcleaning zone, said field having a direction and intensity suflicient tocause toner particles in said background areas of said recording surfaceas Well as toner particles suspended in said zone to electrophoreticallymigrate to said toner layer.

2. A method according to claim 1 including minimizing relative movementbetween said recording surface and said toner layer While said recordingsurface and said toner layer are maintained in contact with each other.

3. A method according to claim 1 wherein said toner particles have anaverage particle size of less than about 1 micron.

4. A method according to claim 1 including removing said surface of saiddonor member from said cleaning zone, depositing a uniform layer oftoner particles on said surface of said doner member and contacting saiduniform layer of toner particles with an undeveloped electrostatic imageon a recording surface of an electrostatographic recording member.

5. A method according to claim 4 including removing substantially alltoner particles from said donor surface prior to depositing said uniformlayer of toner particles.

6. A method according to claim 1 including providing an externallyapplied electric field through said recording surface and said tonerlayer while said recording surface and said toner layer are maintainedin contact with each other.

7. An imaging apparatus comprising a liquid developer container, a donormember having a donor surface positioned within said container, means toprovide a uniform toner layer on said donor surface, means to contact anelectrostatic latent image bearing recording surface of an imagingmember with said donor surface While said donor surface is immersed in aliquid developer, means to separate said recording surface from saiddonor surface, means to maintain said recording surface adjacent to andspaced from said donor surface to form a cleaning zone in said liquiddeveloper therebetween, means to supply an electric field across saidcleaning zone from said recording surface to said donor surface andmeans to remove said recording surface from said cleaning zone.

8. An imaging apparatus according to claim 7 wherein said donor memberhas a continuous donor surface.

9. An imaging apparatus according to claim 8 wherein said donor memberis a continuous belt.

10. An imaging apparatus according to claim 7 wherein the said means toprovide a uniform toner layer on said doner surface comprises electrodespositioned on opposite sides of a portion of said donor member.

References Cited UNITED STATES PATENTS 3,096,198 7/1963 Schaffert 117-373,129,115 4/1964 Clark et al. 117-37X 3,276,896 lO/1966 Fisher 117373,346,475 10/1967 Matkan et a1. 11737X 3,376,852 4/1968 Weiler 118--637WILLIAM D. MARTIN, Primary Examiner E. I. CAB-1C, Assistant ExaminerU.S. Cl. X.R.

